Refactor stores to use one database connection per query

[jochenklar]

Replaces #8. Should be a bit more robust and less complex. I think the performance loss by using separate connections is not relevant to use, Django uses one connection per request by default.

[MyPyDavid]

Does that with MySQLdb.connect(**config.STORE_CONNECTION) actually work?

PS Ok, the MySQLdb.connect has these context methods in it:

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        self.close()

and what about potential rollbacks?

from contextlib import contextmanager

class BaseStore:
    @contextmanager
    def get_cursor_from_connect(self, connect_fn):
        """
        Open a new connection using connect_fn(), yield a cursor,
        commit on success, rollback on exception, and always close.
        """
        conn = connect_fn()
        cur = conn.cursor()
        try:
            yield cur
            try:
                conn.commit()
            except Exception:
                # Some drivers may be autocommit or not support commit -> ignore
                pass
        except Exception:
            try:
                conn.rollback()
            except Exception:
                pass
            raise
        finally:
            try:
                cur.close()
            except Exception:
                pass
            try:
                conn.close()
            except Exception:
                pass

eg in mysql store:

class MysqlStore(BaseStore):

    def connect(self):
        return MySQLdb.connect(**config.STORE_CONNECTION)

    def create_table(self):
        with self.get_cursor_from_connect(self.connect) as cur:
            cur.execute("""
                CREATE TABLE IF NOT EXISTS history (
                    id INT AUTO_INCREMENT PRIMARY KEY,
                    user_identifier VARCHAR(150),
                    project_id INT,
                    messages JSON,
                    created TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
                    updated TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
                    UNIQUE KEY unique_user_project (user_identifier, project_id)
                );
            """)